This invention relates to finishes for glass fabrics that are used to reinforce structures formed from epoxies and like materials. In particular, the invention relates to finishes employing silane based coupling agents for woven glass fibers. More specifically, the invention relates to glass fiber fabric reinforced circuit board laminates.
The need for coupling agents was first recognized in 1940 when glass fibers began to be used as reinforcement in organic resin composite structures. Specific strength to weight ratios of early glass fiber resin composites were higher than those of aluminum or steel, but they lost much of their strength during prolonged exposure to moisture. The interface between such dissimilar materials as an organic polymer and an inorganic glass fiber did not allow the formation of a water resistant bond. A variety of materials have since been developed in an attempt to provide a stable interface under a varying number of adverse environments. These coupling agents can generally be described as molecules which possess two different kinds of reactivity. The siloxane portion of these molecules has reactivity with the glass, while the organic portion of the molecule is tailored to react with organic thermosetting resins used in composite manufacturing. The main function of the coupling agent is to provide a stable bond between two dissimilar surfaces.
The majority of such coupling agents have the general formula:
R CH2 CH2 CH2 Si (OCH3)3 
Where R is a reactive organic group tailored to match the reactivity of the resin system with which it will be used.
The siloxane portion will react with the glass surface as: 
More than one SiOH group may react with the glass surface, or alternatively with other silane molecules to form siloxane oligomers or polymers, which can still provide a link between glass and resin.
Epoxy resins have commonly been used in the manufacture of multilayered laminates for various applications in the electronic, recreation, marine, and aerospace industries. The most common epoxy is formed from epichlorohydrin and bisphenol A. The resin is usually provided in the form of a low molecular weight oligomer, which can be cross-linked with a bi-functional curing agent to result in a solid thermoset polymer. Catalysts are often added to accelerate the reaction with the curing agent. Multi functional epoxies are sometimes added to the resin mix to improve the high temperature resistance of the cured resin. Straight chain polymerization of epoxy can result in a solid material, which is thermoplastic and can be melted. Cross-linking with the curing agent provides a thermoset solid, which does not melt.
To make multilayered laminates, prepregs are first made by pulling glass cloth through a solution of the particular resin system chosen. The glass cloth is impregnated with the resin mix and then proceeds to a heated tower where the solvent is driven off and the resin is partially polymerized to a xe2x80x9cBxe2x80x9d stage. It is important that little, or no cross-links occur, before the resin can melt and flow in the laminating process. The prepreg process is tightly controlled to provide an optimum melt viscosity for lamination.
Prepregs are tested for melt viscosity, resin gel time and resin content. The gel time measurement is widely used in determining the potential reactivity of the prepreg material, as well as, the time available for resin flow in press lamination. Controlling these parameters has been thought to be critical if a void free cured laminate is to be obtained. Gel time is also an indicator of the rate of increase of melt viscosity in press lamination.
A common problem for multilayered laminates, such as circuit board is delamination during wave soldering. The most common cause of delamination during soldering is moisture absorption. The thermal energy imparted to the board in contact with 550xc2x0 F. solder vaporizes any absorbed moisture and the resulting steam pressure forces the laminations apart at the line of the weakest bond. Moisture, which accumulates in even minute voids, is especially likely to produce blistering during the soldering process.
Accordingly, it is one objective of the present invention is to eliminate voids and the resulting entrapped moisture in laminated fiberglass/epoxy composites, so that rupture of the structures will not occur due to the thermal shock of the soldering process.
In the preparation of woven glass fabrics for use in a composite epoxy structure for circuit boards, organofunctional alkoxysilane finishes are applied to heat cleaned fiberglass fabrics from dilute aqueous solutions. The finish content of the dried fabric is typically 0.075% to 0.30% of the fabric weight.
Epoxy resins are usually formulated with difunctional or multi functional curing agents, which provide cross-linking thereby resulting in a thermoset polymer after curing. A catalyst to accelerate the curing is often added to the epoxy formulation. In some cases epoxy resin, curing agent, and catalyst are applied to a glass fiber substrate in solution, and dried at an elevated temperature to remove the solvent. In some applications, it is advantageous to continue heating impregnated substrates to increase the resin molecular weight, and thus its melt viscosity. This is known as a xe2x80x9cBxe2x80x9d stage prepreg. Any significant cross-linking in the B stage prepreg will prevent the resin from melting and flowing in a heated press during consolidation and curing of laminates. The typical epoxy resin formulation for making circuit board prepreg comprises epoxy resin dissolved in an organic solvent, dicyandiamide (dicy) curing agent, and an imidazole accelerating catalyst.
Electrical grade laminates for circuit boards are made by curing layers of epoxy/fiberglass prepreg between copper foil surface sheets. In the laminating process, multiple sheets of epoxy/fiberglass prepreg are placed between copper foil surface sheets. These lay ups are placed between metal laminating plates. A number of these assemblies are stacked to form a book, and each book is placed between heating platens in a multi-opening press. Two competing processes occur as the prepreg is heated in the press. First, the epoxy resin melts, and its viscosity is reduced with increasing temperature. As the temperature rises, the resin begins to polymerize and increase in viscosity. Finally the resin is sufficiently cross-linked that it gels and can no longer flow. Consolidation of the laminate must be completed before the resin gels. Complete cure is achieved with additional time in the press and increased temperature. The two processes must be carefully balanced to insure a void free laminate with good thickness control, and minimize resin loss at the laminate edges. If the resin gels too soon, there may not be sufficient flow to remove solvent or air trapped in the capillaries between individual filaments in a fiber bundle. Minute voids in the capillary spaces of the cured laminate are often referred to as silver streaks.
In circuit board manufacturing, the copper clad boards are coated with a photosensitive acid resist. The desired circuitry is then photo printed on the copper. The board is then subjected to a hot acid bath to remove the unwanted copper. Holes are drilled for mounting surface components, or for establishing electrically conductive connections between circuits on both sides of the board.
The holes are then electroplated. Finally, the board with its assembled components is floated across a 550xc2x0 F. molten solder bath. Any moisture, which has been absorbed into a void or silver streak during the wet processing of the board, will cause it to blister. If a void stretches between two adjacent through holes, it may cause a short circuit in the finished assembly.
As with most manufacturing processes, it is desirable to maximize the productivity of capital equipment. For some applications, a high laminate glass transition temperature is required. These objectives can be achieved by increasing the catalyst, the curing agent, or the processing temperatures, individually or in combination. These speeded up processes are difficult to control, and have a very narrow processing window for quality production. Any premature gelling of the resin in the prepreg xe2x80x9cBxe2x80x9d stage or in lamination will result in scrap laminates. Capillary voids cannot be seen until the copper foil is etched. Even then, they are often hard to see. If silver streaks are undetected in the laminates in early stages of circuit fabrication, the cost of scrap escalates. As a quality check, acid is used to remove all of the copper foil from one or more laminates of a production lot to check for silver streaks. This does not preclude that some laminates in the lot may have silver streaks. Understandably, circuit board laminators, and their customers, are anxious to have the assurance of materials and processes, which eliminate all silver streaks.
One method of increasing the size of the processing window, is to add a weak acid, such as boric acid, succinic acid, citric acid, benzoic acid or maleic acid, to the epoxy resin mix to complex with the catalyst, and make it latent until a desired curing temperature in the range of 190-210xc2x0 C. is reached in the press, at which time the cure is very rapid and complete. Such a process is described in U.S. Pat. No. 5,308,895 and U.S. Pat. No. 5,314,702. The objective of this process, is to have all of the resin in the prepreg gel at the same time and temperature and cure very rapidly.
Silver streaks have been an occasional quality problem in electrical laminates in the past, but the move to faster curing resin systems has greatly increased the severity and cost of the problem. While the latent catalyst approach of the before mentioned patents sufficiently controls the gel time of free resin on the surface of the prepreg layers and between yarns, there may be insufficient gel time or too rapid an increase in melt viscosity for the inherently slower capillary flow within the yarn bundles to eliminate capillary voids or silver streaks. Lengthening the overall gel time of the resin to accommodate greater capillary flow will result in excessive edge loss and lack of thickness control. Since the glass fiber fabric is typically only 30% to somewhat less than 50% of the volume of typical electrical prepreg, ensuring sufficient flow in the yarn capillaries to eliminate voids without excessive outflow of the free resin presents a serious problem.
Furthermore, some of the best finishes for obtaining superior laminate properties contain silanes with amine functional groups. These amino-functional silanes have primary or secondary amine groups, which can react with, or catalyze, epoxy resins at a lower than desired press temperatures. Amine functional silanes are advantageous for finishing fiberglass fabrics. Because of their cationic nature, they are attracted to the anionic glass fiber surface, and provide a more evenly distributed finish.
Accordingly, a broad objective of the present invention is to eliminate the silver streak problem.
One specific objective of the present invention is to selectively control the gel time of resin in the capillaries between the filaments of the yarn bundles making up the fiberglass fabric.
A second objective of the invention is to provide a coupling agent finish for fiberglass fabrics to independently control the gel time of resin in the capillaries and melt viscosity.
Still another objective of the invention is to provide a simple test method to predict, in the laboratory, the degree capillary gel time control provided by any given fabric finish.
These and other objectives of the invention are achieved by the present invention, which will be better understood by the summary of the invention and detailed description, which follows below:
It has surprisingly been found that the foregoing objective can be achieved by incorporating a catalyst inhibitor in the finish applied to glass fibers and glass fabric, so that the gel time of a polymeric resin is selectively lengthened as the resin flows around individual glass filaments and the capillary spaces between them. The longer time to gel within the yarn bundles allows the resin to flow completely around filaments eliminating capillary voids or silver streaks, while the free resin retains its faster gel time preventing excessive edge resin flow and maintaining thickness control. This benefit is surprisingly achieved without any increase in cure time in the press.
Accordingly, in one primary aspect the invention is a composition and method for selectively controlling the gel time of an epoxy resin in the capillary regions of a glass fabric. Another aspect of this invention is a novel finish comprising an epoxy reactive organosilane coupling agent and catalyst inhibitor capable of selectively lengthening the flow time within the capillaries of a glass fabric. Still another aspect of this invention is a laboratory test to determine the relative degree of lengthening of said gel time.
A further aspect of the invention, is a method of selectively lengthening the gel time of an epoxy resin within the yarn bundles of a fiberglass fabric comprising the steps of providing cleaned woven glass fabric, providing a silane finish bath for the fabric, adding boric acid or other weak acid to the bath, immersing the fabric in the bath, drying the fabric, and subsequently impregnating the fabric with an epoxy resin mix whereby the gel time within the yarns is selectively lengthened.
In another aspect the finish of the invention includes at least one cationic amino-functional alkoxysilane coupling agent and a weak acid that can complex with the amino function to provide a latent reactivity with epoxy resin, whereby during lamination under heat and pressure the finish selectively provides a longer resin flow time and a slower increase in resin viscosity. The finish may include an additional alkoxysilane coupling agent having a critical surface tension greater than that of the epoxy resin to promote resin wetting of the finished fabric. The weak acid in the finish may be predetermined to complex with the catalysts in selected resin formulations.
The invention also includes the method of making glass fiber reinforced articles of polymeric materials including circuit boards and the like products where it is important that the polymeric material substantially fills the voids between fiber filaments.